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Abstract
This paper presents a simple coefficient method of calculating average vertical illumination. Two new coefficients are introduced, the vertical coefficient of utilization (VCU) and the directed coefficient of utilization (DCU). The vertical coefficient of utilization predicts the average of all possible vertical footcandle readings over a three-dimensional grid within a room. The directed coefficient of utilization, based on balanced horizontal and vertical illumination, is a relative measure of three-dimensional lighting effectiveness; and it can be used to recommend wide or narrow beam luminaires according to room parameters.